Markdown filter: add an option to toggle extraction of image alt text

[Chase Tingley]

Markdown images can support both alt text and a title:

![alt text](https://github.com/adam-p/markdown-here/raw/master/src/common/images/icon48.png "Logo Title Text 1")

Currently we extract both alt text and title. Not everyone localize alt text, so we should make this an option. The text will be extracted by default.

[Chase Tingley]

Fix issue #651 - add an option to toggle markdown alt text extraction

→ <<cset b941e20ef0d2>>